Banks are subject to far more stringent record keeping requirements than most other companies. Microsoft is not bound by rules similar to the article you reference.

That said, there are some categories of information that Microsoft would have to retain. In particular, once you reasonably anticipate imminent litigation you have a duty to retain related information.

Banks are far more stringent... For the rank and file, because even low level employees in banks can access private information that allow them to insider trade.

For the CEOs, they were already under intense scrutiny for every single trade, so it makes more different.